1. Field of the Invention
The present invention relates to a multistage transmission in which a plurality of drive gears and driven gears are rotatably supported by respective parallel gear shafts in a constant-mesh state for each speed-change stage.
2. Description of Background Art
A conventional constant-mesh type multistage transmission is such that one of drive gears and driven gears is secured to a gear shaft, the other is rotatably supported by a gear shaft, and a gear engaged with a rotating shaft is switched from the rotatable gears by engaging means for executing shifting.
Engaging means are proposed each of which generally brings a driven gear into engagement with a driven gear shaft, the driven gear being rotatably supported by the driven gear shaft (see e.g. Japanese Patent Publication No. Sho 45-35687).
The engaging means of the constant-mesh type multistage transmission disclosed in Japanese Patent Publication No. Sho 45-35687 is such that a cylinder body formed with axially extending elongated grooves and with widened holes circumferentially widened at the central position of each of the elongated grooves is externally provided on a driven gear shaft so as to be movable in the axial direction and a plurality of driven gears are rotatably supported on the circumference of the cylindrical body.
Rollers installed in incision grooves which are formed at portions of the driven gear shaft corresponding to the driven gears so as to be flatly cut away are loosely fitted in line into the elongated grooves of the cylindrical body except the widened holes.
Among the driven gears rotated through constant-mesh with the drive gears, the driven gears in which the rollers in the elongated groove are in non-contact on the inner circumferential surfaces of the driven gears do not transmit rotation, and only the driven gears in which the rollers in the widened holes are in contact and engagement with the inner circumferential surfaces of the driven gears transmit rotation.
That is to say, since the driven gear located at a position corresponding to the widened hole transmits the rotation to the driven gear shaft, shifting can be executed by axially moving the cylindrical body.
The widened hole adapted to bring the roller into engagement with the driven gear is formed with inclined surfaces at respective inside edges. This makes it easy for the roller to circumferentially move along the inclined surfaces when the cylinder body moves. However, the roller is firmly restrained in place between the incision groove and the inner circumferential surface of the driven gear. Therefore, a significantly large force is needed to release this engagement so that it is not easy to smoothly move the cylindrical body. A shift clutch is needed depending on circumferences.
During shifting, there is a loss of time needed for the switching as described below. The cylindrical body is moved to allow the widened holes to disengage the rollers from driven gear. Then, the widened holes reach the next rollers, which circumferentially move and engage with the adjacent driven gear.
In addition, during shifting, when the engagement of the rollers with the driven gear is released, escape of a driving force occurs and a large shift shock occurs at the time of engaging with the adjacent driven gear. Thus, smooth shifting is difficult.
Another configuration (included in an application previously filed by the present Applicant) is provided herein as another example illustrating the means for engaging the gear with the gear shaft in a conventional constant-mesh type multistage transmission. In this example, cam rods in slidable contact with the hollow inner circumferential surface of the gear shaft are axially slid to operate an engaging member provided in the gear shaft. This operation of the engage member establishes engagement with the gear and releases the engagement therefrom (Japanese Patent Application No. 2008-093699).
The cam rods in slidable contact with the hollow inner circumferential surface of the gear shaft in the multistage transmission disclosed by Japanese Patent Application No. 2008-093699 are generally formed by circumferentially dividing a circular cylinder into four sections. In addition, locking is configured to prevent the relative rotation of each cam rod relative to the gear shaft in such a manner that four projecting ridges formed on the inner circumferential surface of the gear shaft grip the corresponding cam rods from both lateral edges thereof.
The cam rods are elongate members formed by circumferentially dividing the circular cylinder into four sections, i.e., formed like sectors in cross-section. Therefore, they cannot be manufactured with ease. In addition, it is not easy to form the projecting ridges for locking on the hollow inner circumferential surface of the gear shaft.
Still another configuration (included in an application previously filed by the present Applicant) is provided herein as still another example illustrating the means for engaging the gear with the gear shaft in a conventional constant-mesh type multistage transmission. In this example, lost motion mechanisms are assembled into shift drive means which executes shifting by switching and driving engaging means provided between a plurality of gears and a gear shaft to establish engagement therebetween (Japanese Patent Application No. 2008-093701).
The shift drive means of the multistage transmission disclosed in Japanese Patent Application No. 2008-093701 is as below. A control rod disposed at the hollow central axis of the gear shaft is in slidable contact with the insides of two kinds of cam rods in axially movably slidable contact with the hollow inner circumferential surface of the gear shaft. The lost motion mechanisms are disposed at both ends of the cam rods. The lost motion mechanisms are each connected with a corresponding one of the cam rods.
The lost motion mechanism is such that a spring is interposed between the control rod and each cam rod so as to axially act thereon for simultaneous movement. The lost motion mechanisms are compactly housed into the hollow of the gear shaft.
However, since the lost motion mechanisms are arranged at both the ends of the cam rods, the gear shaft is increased in length to increase the axial width of the transmission, which leads to the enlarged transmission.